Simple analytic approximate expressions for the solution to the Poisson-Boltzmann equation around a spherical particle coated with an ion-penetrable polyelectrolyte layer in a salt-free medium containing counterions only are derived. The results of the calculation of the potential distribution using the approximate solution are found to be in good agreement with exact numerical results. It is shown that as in the case of a charged rigid particle, there is a certain critical value of the particle charge, separating two cases, that is, the low-particle-charge case and the high-particle-charge case. In the low-charge case the potential is essentially the same as if counterions were absent and thus the potential is proportional to the particle charge. In the high-charge case counterion condensation occurs in the polyelectrolyte layer region, so that the dependence of the potential on the particle charge is considerably suppressed. (C) 2003 Elsevier Inc. All rights reserved.